In order to find and produce hydrocarbons e.g. petroleum oil or gas hydrocarbons such as paraffins, naphthenes, aromatics and asphaltics or gases such as methane, a well may be drilled in rock (or other) formations in the Earth.
After the well bore has been drilled in the earth formation, a well tubular may be introduced into the well. The well tubular covering the producing or injecting part of the earth formation is called the production liner. Tubulars used to ensure pressure and fluid integrity of the total well are called casing. Tubulars which bring the fluid in or from the earth formation are called tubing. The outside diameter of the liner is smaller than the inside diameter of the well bore covering the producing or injecting section of the well, providing thereby an annular space, or annulus, between the liner and the well bore, which consists of the earth formation. This annular space can be filled with cement preventing axial flow along the casing. However if fluids need to enter or leave the well, small holes will be made penetrating the wall of the casing and the cement in the annulus therewith allowing fluid and pressure communication between the earth formation and the well. The holes are called perforations. This design is known in the Oil and natural gas industry as a cased hole completion.
An alternative way to allow fluid access from and to the earth formation can be made, a so called open hole completion. This means that the well does not have an annulus filled with cement but still has a liner installed in the earth formation. The latter design is used to prevent the collapse of the bore hole. Yet another design is when the earth formation is deemed not to collapse with time, then the well does not have a casing covering the earth formation where fluids are produced from. When used in horizontal wells, an uncased reservoir section may be installed in the last drilled part of the well. The well designs discussed here can be applied to vertical, horizontal and or deviated well trajectories.
To produce hydrocarbons from an oil or natural gas well, a method of water-flooding may be utilized. In water-flooding, wells may be drilled in a pattern which alternates between injector and producer wells. Water is injected into the injector wells, whereby oil in the production zone is displaced into the adjacent producer wells.
A horizontal, open hole completion well can comprise a main bore or a main bore with wanted side tracks (fishbone well) or a main bore with unwanted/unknown side tracks.
Further, a horizontal, open hole completion well may, when producing hydrocarbons (producer well) or when being injected with water (injector well) be larger than the original drilled size due to wear and tear.
Additionally, horizontal, open hole completion wells can have wash outs and/or cave ins.
During the different phases of establishing a well in a formation of the earth and/or during subsequent carbohydrate production, a variety of downhole apparatus may permanently or temporarily be installed in the well.
Published international patent application WO 98/12418 discloses an elongate autonomous robot which is released downhole in an oil and/or gas production well by means of a launching module that is connected to a power and control unit at the surface. The elongated robot is equipped with sensors and arms and/or wheels which allow the robot to walk, roll or crawl up and down through a lower region of the well.
A downhole apparatus may thus comprise several sensors and/or electrical or hydro-mechanical components that produce sensor signals and/or require control signals as input. Furthermore, a downhole apparatus may comprise a plurality of movable parts that move relative to each other during operation.
Operation of a downhole apparatus is thus a complex operation and requires complex, fragile and expensive equipment. Recovery of a defective downhole apparatus may be a complicated and costly operation that also causes delays in the production of a hydrocarbon reservoir. It is thus generally desirable to allow efficient and reliable control of the relative movements of the movable parts and/or the electrical and/or hydro-mechanical components installed in a downhole apparatus and/or to allow efficient and reliable retrieval of sensor data in a downhole apparatus with a plurality of movable parts, even under difficult environmental operating conditions, such as under high pressure, e.g. at seabed, in areas with high levels of radiation, e.g. radioactive radiation, exposure to humidity, oil, mechanical impact and/or the like.
The spatial constraints of a downhole bore further limit the degrees of freedom for designing downhole apparatus that operate efficiently and reliably.